Digital Subscriber Line (DSL) lines are often underutilized, at times transmitting little or no data, which is an inefficient use of power. It may take a minute or more to re-start a DSL line that has been completely turned off. Thus, completely turning off a DSL line to save power often causes undesirable delay in re-starting the DSL line. There are many times when it would be desirable to transmit data on the DSL line at low bit-rates sufficient, for example, to transmit keep-alives and/or possibly certain types of data traffic, such as low levels of Internet data traffic, or Voice over Internet Protocol (VoIP) data traffic, with very low power consumption.
The low-power states used in current DSL line transmission standards such as ADSL2 and ADSL2plus [ITU-T standards G.992.3 and G.992.5] can save power on DSL lines by lowering the transmit power and the transmit power spectral density (PSD), and using lower bit loading to carry traffic on the DSL line at a lower bit rate. This low-power state is referred to “L2-mode.” L2-mode can lower transmit power by up to 31 dB total maximum aggregate transmit power reduction in L2-mode (L2-ATPRT).
FIG. 2 illustrates a typical DSL environment 200 in which two DSL lines are located within the same cable. In particular, the telephone local loop or cable 112 is coupled from Digital Subscriber Line Access Multiplexer (DSLAM)/AN 114 to one or more TU-R units, for example, TU-R 122a and TU-R 122b. Multiple DSL lines may exist within the cable, for example, DSL line 112a coupled from TU-C 142a to TU-R 122a, and nearby DSL line 112b coupled from TU-C 142b to TU-R 122b. DSL lines adapt to the channel and noise levels at start-up, and if the noise subsequently increases by a level at or above the signal to noise ratio (SNR) margin then the DSL line can become unstable and resynchronize. Much of the noise is “crosstalk”, such as depicted at 205, from other DSL lines using the same cable, in the same loop, or in the same binder, caused by transmitting signals in both the downstream direction 210 and/or the upstream direction 215. A potential problem with low power states on a DSL line is that they can create time-varying crosstalk, also referred to as fluctuating crosstalk, non-stationary crosstalk, or short-term stationary crosstalk in nearby or neighboring DSL lines. Raising and lowering the transmit power or transmit PSD levels of signals transmitted on the DSL line also raises and lowers the crosstalk received by nearby DSL lines. If the crosstalk abruptly or suddenly increases by a large amount then one or more of these nearby DSL lines may need to resynchronize; such “instability” is highly undesirable. Resynchronization can be avoided by certain methods including slowly exiting from the low-power states, as described in U.S. patent application Ser. No. 12/700,892 entitled Apparatus, Systems and Methods for DSM Energy Management, assigned to the assignee of this patent application. Such methods allow nearby DSL lines sufficient time to adapt to the higher crosstalk level induced by increasing the transmit power and transmit PSD levels of signals transmitted on a DSL line via seamless rate adaptation (SRA) or other techniques. However, the current DSL standards and DSL equipment do not allow slow exit from low-power mode; it can only be done rapidly, as set forth in ITU-T G.992.3 and G.992.5, and Broadband Forum TR-202, which creates instability in nearby DSL lines. Further, while low-power states may be gradually entered, this does little to help stability. Power consumption can also be reduced by suspending data transmission on a DSL line entirely for brief moments when there is no data traffic to transmit, or by queuing the data traffic for transmission at a later time. This suspension of data traffic on the DSL line is referred to as “discontinuous operation.” With reference to FIG. 3, discontinuous operation 300 involves the DSL transceiver's transmitter turning “ON” and transmitting at some times (305, 315, 325 and 335), and turning “OFF” the transmitter at other times (310, 320, and 330). The duration of the ON and OFF times could be any possible value. Alternatively, to ensure synchronization of the DSL line or to track channel variations on the DSL line the maximum OFF time and/or the minimum ON time may be specified. FIG. 3 illustrates at 340, 345 and 350 turning on the transmitter, for example, when traffic arrives at the transmitter, when the amount of traffic arriving at the transmitter increases, when a queue maintained by the transceiver in which traffic to be transmitted by the transmitter reaches a certain threshold, or when a timer reaches a certain time or time out value. When there is no traffic to transmit, the transmitter may be turned OFF. Turning the transmitter OFF may even involve briefly turning off DSL transceiver functionality entirely, which could enable large power savings. Discontinuous operation is generally performed in real-time, in sub-second on/off duration times. Discontinuous operation can, however, cause very large and frequent fluctuations in crosstalk and can cause widespread instability problems in neighboring DSL lines. Also, a DSL line that doesn't transmit for a long time may itself lose synchronization and not properly track the channel.